1. Field of the Invention
This invention relates to a multi-layer information medium which has at least two information-storing layers such as recording layer
2. Prior Art
There is a growing need for an optical disk having a higher density and a higher capacity. DVD (Digital Versatile Disk) is already commercially available, and the DVD has a storage capacity of about 4.7 GB per single side which is about seven times larger than the compact disk. Technologies enabling further increase in the amount of information recorded have been actively developed.
Technologies that have been used for increasing the recording capacity of an optical disk include use of a recording/reading beam having a shorter wavelength, use of an objective lens having a higher NA (numerical aperture) in the optical system irradiating the recording/reading beam, increase in the number of recording layers, and multi-value recording. Among these, three-dimensional recording by increasing the number of recording layers enables remarkable increase in the recording capacity at low cost compared to the use of a shorter wavelength or use of a lens with a higher NA. The multi-layer information medium wherein the three-dimensional recording has been enabled is described, for example, in Japanese Patent Application Kokai (JP-A) 198709/1997, and JP-A 255374/1996 discloses a medium wherein a rewritable information storage layer and a read only information storage layer are laminated.
A multi-layer information medium has a plurality of data layers each storing the recorded information, and resin layers transparent to the recording/reading beam are formed between the adjacent data layers. The resin layer is preferably formed from a coating comprising an active energy radiation-curable (actinic-curable) resin such as UV-curable resin since use of such resin enables formation of a relatively homogeneous layer at a high productivity.
When the resin layer is formed of active energy radiation-curable resins, however, the medium may become deflected due to shrinkage upon curing. Deflection can also occur when the medium is stored in a hot humid environment. Once the medium is deflected, and in particular, once the medium is deflected with twisting, the medium will experience increased facial runout as well as increased acceleration of facial runout during its rotation, and as a result, errors can frequently occur upon reading, and excessive deflection can cause the media to be unreadable. A multi-layer information medium is highly susceptible to such deflection associated with twisting since the resin layer of substantial thickness should be deposited in order to suppress the crosstalk between adjacent data layers, and two or more resin layers should be deposited when the medium is to be provided with three or more data layers.
In the case of the medium having a single recording layer formed on a substrate, the shape of the grooves (guide grooves) formed in the resin substrate will be transferred to the recording layer. In contrast, in the case of a medium wherein two or more data layers are formed on the substrate with an intervening relatively thick resin layers between the recording layers, it is quite difficult to transfer the shape of the grooves formed in the substrate to all of the recording layers since the groove depth is about 100 nm at most for optical reasons while the distance between the recording layers is far more than that. As a consequence, formation of the grooves in the transparent resin layer by photopolymerization (2P) process will be required as described, for example, in the JP-A 198709/1997 and an eminent increase in the production cost is invited.
An object of the present invention is to provide a multi-layer information medium wherein the deflection of the medium associated with twisting is reduced, and another object of the present invention is to provide such multi-layer information medium at low cost.
Such objects are attained by the present invention as described in (1) to (7), below.
(1) An optical information medium comprising a substrate or a pair of substrates; at least two information-storing layers for storing recorded information and/or tracking servo information disposed on said substrate or between said pair of substrates, wherein at least one information-storing layer is recorded or read by the recording beam or the reading beam which has passed through other information-storing layer(s); and resin-containing intermediate layers on at least one side of each of said information-storing layers, wherein
at least one of said intermediate layers is a cured intermediate layer containing an active energy radiation-curable resin, and at least one of said intermediate cured layer(s) has a tensile elongation at break of 15 to 200% and a tensile modulus of 20 to 1,000 MPa.
(2) An optical information medium according to the above (1) wherein the medium has one substrate, and one surface of the medium comprises a protective layer comprising a resin, said protective layer having a tensile modulus higher than that of said cured intermediate layer(s).
(3) An optical information medium according to the above (2) wherein the medium has at least two cured intermediate layers, and the cured intermediate layer which is farther from the substrate has a higher tensile modulus.
(4) An optical information medium according to the above (2) or (3) wherein said protective layer comprises an active energy radiation-curable resin.
(5) An optical information medium according to any one of the above (1) to (4) wherein said intermediate layer includes a resin sheet.
(6) An optical information medium according to the above (5) wherein said resin sheet has a tensile elongation at break of 15 to 200%.
(7) An optical information medium according to any one of the above (1) to (6) wherein said information-storing layer comprises at least one data layer for storing recorded information and at least one servo layer for storing servo information which is formed independently from said data layer.